1. Field of the Invention
The present invention generally relates to an electrolyte, particularly to an electrolyte body or layer for use in a cell that flows or rather transfers ions such as oxygen ions, lithium ions, sodium ions between the electrodes of the cell. Specifically the invention provides a gas sensor with a solid electrolyte for detecting or rather measuring a concentration of a specific gas component such as O2, CO2, NOx, HC, H2O and H2. More specifically, this invention provides a gas sensor with an electrochemical cell using an oxygen ion conductive solid electrolyte that is capable of transferring or conducting oxygen to thereby detect a specific gas component existing in an exhausted gas emitted from an internal combustion engine, and a method for fabricating the gas sensor. Further, the invention provides a new oxygen-ion conductive solid electrolyte material and a strenuous gas-sensor structure using the same.
2. Description of the Related Art
Conventionally, various gas sensors using a solid electrolyte such as zirconia have been proposed for internal combustion engine control. For instance, a so-called lambda sensor that employs a cylindrical and bottom-closed solid electrolyte has been widely used for detecting oxygen in a gas exhausted from an internal combustion engine. On the other hand, a so-called thick-film gas sensor that utilizes a thick electrolyte film or rather layer formed on a ceramic substrate or rod as a sensing element has been proposed, which sensor enables prompt activation of a gas-sensing mechanism as compared to the lambda sensor. This is because its heat-propagation efficiency is comparatively high compared to the lambda sensor. The thick-film gas sensor may include an insulating ceramic substrate or rod in which a heating wire is embedded and insulated from the electrolyte film, and which substrate is co-fired with the electrolyte film so as to form an integral or unitary ceramic laminate as a gas sensor.
In a conventional process of fabricating the thick-film gas sensor, a green or rather unfired oxygen-ion conductive solid electrolyte layer including zirconia particles therein and unfired metal electrode wires formed thereon is superposed on an unfired alumina substrate, and then the layer and the substrate are co-fired to form the unitary laminate. However, this process encounters a problem in that since the alumina substrate and the zirconia differ in thermal coefficient and thermal expansion and the zirconia undergoes a phase transition with firing temperature variations, a volume change and/or thermal stress is induced in the laminate. This causes difficulties in attaining a high-quality oxygen-ion conductive solid electrolyte layer of zirconia firmly bonded on the alumina substrate without losing the required performance by co-firing. Otherwise, cracks are induced in the resultant oxygen-ion conductive solid electrolyte layer formed on the laminate in a thermal cycle environment ranging, e.g., from −20° C. up to 1100° C. (hereinafter referred to as “thermal cycle”).
Suppression of cracking in the laminate and firmly bonding the oxygen-ion conductive solid electrolyte layer thereon are disclosed in Japanese Patent Application Laid-Open (kokai) Nos. 61-51557, 61-172054, and 6-300731. However, these disclosures are still unsatisfactory in bringing about a good solid electrolyte ceramic that can be used as a thick film layer of a gas sensing element formable with an insulating ceramic substrate or rod.